There are many portable electronic device applications wherein a portable electronic device requires low power to support its operation. Calculators and automobile door lock and security system actuating transmitters are just a couple of examples of such portable device applications. In the past, battery power has been utilized to support the operation of such devices. More recently, portable calculators, instead of being powered by a battery, have been powered photovoltaic cells which generate power for the calculators responsive to incident visible light.
One problem associated with battery power is that batteries are depletable power sources. Hence, when a battery becomes depleted, it must either be replaced or recharged. Such replacement or recharging is not always convenient.
Photovoltaic cells, as is well known, require incident light for providing electrical energy. Hence, such devices cannot be relied upon at all times to provide electrical power, as for example, when such devices are in the dark.
From the foregoing, it can be seen that portable power sources of the prior art exhibit numerous disadvantages. Depletable power sources, such as batteries, eventually require replacement or recharging. Photovoltaic cells do not provide electrical current under all circumstances.
Other forms of portable power sources are also known. For example, in the RF tag field, at least one RF tag is known which utilizes an active device in the form of an RF transmitter which transmits an RF signal and is powered by a tuned circuit which converts RF energy at a predetermined frequency to sufficient electrical energy to power the transmitter for a very short period of time, on the order a few milliseconds. In this application, the tuned circuit on the RF tag charges a capacitor when it receives an RF signal at the predetermined frequency. When the capacitor is charged to an appropriate level, the transmitter is energized and transmits its RF signal for just a few milliseconds until the charge on the capacitor is depleted.
While such an arrangement is sufficient for its intended purpose, for use in an RF tag system employed, for example, for access control or automatic toll debiting, it is not suitable for general application to portable electronic devices. The arrangement provides power for only a short period of time and the tuned circuit generates power responsive to receipt of RF energy transmitted at a specific frequency. As a result, the amount of power generated is extremely low, severely limiting the range of the transmitted signal. Further, such an arrangement requires two separate antennas. One antenna must be relatively large for generating sufficient electric current to charge the capacitor. The second antenna is utilized as the transmitting antenna. With the need for two separate antennas, and especially with one antenna being rather large in dimension, RF tags of this type have in turn been relatively large in dimension.
Hence, there is a need in the art for an improved power source for portable electronic device applications. Such an improved power source preferably continuously generates electrical current under a variety of conditions for the storage of electrical energy for use at any time by the portable electronic device with which it is associated. The power source preferably generates electrical current for storage responsive to a plurality of different forms of incident energy such as electromagnetic energy (RF energy, visible light, etc.) and vibrational energy such as acoustic energy. There is also a need in the art for an improved RF tag which includes a portable receiver and transmitter which are powered by such an improved power source.